Insulation calculator (R-value, bags, and cost)

How much insulation do I need? Bags and square footage

For blown insulation, the answer that matters at Home Depot or Lowes is not the R-value, it is the bag count. The calculator above does both. Bag coverage scales inversely with thickness: one bag of cellulose covers about 50 square feet at R-19 depth but only about 20 square feet at R-49 depth, because the same volume of material gets spread thinner in the higher-R install. Industry coverage rates (Greenfiber, Owens Corning, Knauf published spec sheets):

• Blown cellulose at R-49 (14 inches): about 10 square feet per 25 lb bag. A 1,000 square foot attic needs roughly 100 bags at $12 to $15 per bag, or about $1,200 to $1,500 in material.
• Blown cellulose at R-30 (8.5 inches): about 17 square feet per bag. Same 1,000 square foot attic needs about 60 bags, $700 to $900 material.
• Blown fiberglass at R-49 (20 inches): about 15 square feet per bag, ~67 bags per 1,000 sqft at $30 to $40 each, $2,000 to $2,700 material.
• Blown fiberglass at R-30: about 35 square feet per bag, ~30 bags per 1,000 sqft, $900 to $1,200 material.

Always add 5 to 10 percent for settling, gaps around the eaves, and machine spillage. The calculator above bakes in a 5 percent waste factor on bag counts. Cellulose settles 15 to 20 percent over the first five years, so to hold R-49 long term you install closer to R-58 at install date. Blown fiberglass settles less, around 5 to 10 percent.

DIY versus pro installed: the cost split that decides the project

Blown insulation is one of the few major home upgrades where DIY makes real financial sense. Big-box rental machines (Owens Corning AttiCat, Cocoon Hopper, Insulmax) are free with a minimum bag purchase (usually 20 bags). The job for a 1,000 square foot attic is one long Saturday with one person feeding bags and another with the hose, plus another hour to vacuum out drop cloths. The DIY cost is usually 40 to 50 percent of pro installed.

Typical installed-cost ranges (calculator returns both numbers):

• Blown cellulose R-49 on 1,000 sqft attic: $1,200 to $1,500 DIY material vs $2,200 to $2,700 pro installed.
• Blown fiberglass R-49 on 1,000 sqft attic: $2,000 to $2,700 DIY material vs $3,600 to $4,900 pro installed.
• Fiberglass batts R-19 wall cavity for 1,000 sqft of wall: $2,500 to $3,000 DIY vs $5,000 to $6,000 pro installed (batts take real labor; less savings than blown).
• Open-cell spray foam R-21 wall cavity for 1,000 sqft: $4,000 to $5,000 DIY (rental kit) vs $5,000 to $6,500 pro installed (DIY foam is risky; pro is usually worth the markup).
• Closed-cell spray foam R-21 wall cavity: $7,500 to $9,500 DIY (rental kit) vs $9,000 to $11,500 pro installed.

Pro install also includes blower-door testing on most ENERGY STAR qualified jobs, IR scan of the finished install, and warranty coverage on the workmanship. For attic blown installs that is usually overkill; for spray foam in a finished wall cavity it is essential because a bad foam job is nearly impossible to remove without tearing out drywall.

IECC 2021 minimum R-values by climate zone

The International Energy Conservation Code Table R402.1.3 publishes minimum R-values by US climate zone for new residential construction. The calculator checks your layered assembly against the right code minimum for the project area you selected:

• Zone 1 (Tampa, Brownsville): ceiling R-30, wall R-13, floor R-13
• Zone 2 (San Antonio, Tucson, San Diego coast): ceiling R-49, wall R-13, floor R-13
• Zone 3 (Birmingham, Las Vegas, New Orleans): ceiling R-49, wall R-20 (or R-13 + R-5 continuous), floor R-19
• Zone 4 (Tulsa, Washington DC, Raleigh): ceiling R-60, wall R-30 (or R-20 + R-5 ci), floor R-19
• Zone 5 (Cincinnati, St Louis, Salt Lake City): ceiling R-60, wall R-30, floor R-30
• Zone 6 (Sioux Falls, upstate New York): ceiling R-60, wall R-30, floor R-30
• Zones 7-8 (Duluth, Fairbanks): ceiling R-60, wall R-30, floor R-38

These are minimums for code compliance, not optimums for energy savings. Houses built to ENERGY STAR or Passive House standards typically run R-60 to R-80 ceilings and R-30 to R-40 walls even in moderate climates. The diminishing-returns curve says the code minimum is the right target for most retrofits and the optimal-energy number is the right target for new construction where you have one shot at the assembly.

How to pick which insulation material goes where

Different parts of the building call for different insulation materials. The calculator's add-on recommendation picks candidates that fit the project area:

• Attic floors: blown cellulose or blown fiberglass. Cheapest material per R, easy to add in any depth, no framing constraints. R-49 to R-60 of blown cellulose runs $1 to $1.50 per square foot installed.
• Wall cavities: fiberglass batts, mineral wool batts, or open-cell spray foam. Drill-and-fill blown-in is also common for retrofit. Closed-cell spray foam works but is usually overkill for cavities.
• Cathedral ceilings and roof decks: closed-cell spray foam (when you need maximum R per inch) or rigid polyiso continuous over the deck.
• Basement walls and crawlspaces: XPS, closed-cell foam, or polyiso. All three are vapor retarders, which is what you want against masonry walls. Never use fiberglass batts against a basement wall. They trap moisture and grow mold.
• Continuous exterior insulation: XPS or polyiso. Polyiso wins on R per inch but underperforms in cold weather, so XPS is the safer choice in zone 6 and colder.

R-values of common insulation materials, per inch

Different insulation materials have different R-values per inch of thickness. The R-per-inch values published by the DOE Insulation Fact Sheet (DOE/CE-0180) and used by the calculator above:

• Fiberglass batt: R-3.2 per inch. The cheapest option. Easy DIY install, but gaps and compression dramatically reduce installed performance.
• Blown cellulose: R-3.5 per inch. Recycled paper with borate fire retardant. Better air-blocking than blown fiberglass because it conforms to gaps. Settles 5 to 10 percent over a decade.
• Mineral wool batt: R-3.2 per inch. Fire and pest resistant. Stiffer than fiberglass which makes it more forgiving of installation gaps.
• Open-cell spray foam: R-3.7 per inch. Lower R than closed-cell but acts as its own air sealer. Vapor-permeable, so it can dry in either direction.
• Closed-cell spray foam: R-6.5 per inch. Highest R-per-inch of any common residential insulation. Also functions as a vapor retarder. Most expensive material per square foot but the only choice when space is at a premium.
• XPS rigid foam: R-5 per inch. Pink or blue boards. Common as continuous exterior insulation outside sheathing.
• EPS rigid foam: R-3.9 per inch. White beadboard. Cheapest rigid foam.
• Polyiso rigid foam: R-6.2 per inch. Highest R-per-inch rigid foam. R-value drops 10 to 20 percent in cold weather, so it underperforms in roof and exterior wall applications in cold climates.

Vapor retarders: the gotcha that ruins more insulation jobs than any other

Wherever insulation goes, moisture management has to go with it. The IRC R702.7 requires Class I or Class II vapor retarders on the warm (interior) side of walls in climate zones 5 and colder. Kraft-faced batts and smart membrane are Class II. Polyethylene sheeting is Class I. Closed-cell spray foam is its own Class II retarder, which is one reason it works in basement walls without a separate barrier.

Getting this wrong causes condensation inside walls and rot. The calculator flags the requirement automatically when you pick a wall assembly in zones 5 through 8. Air sealing the warm side (caulk at top and bottom plates, can foam around penetrations, gasketed outlets) is just as important as the retarder itself. Most of the moisture that gets into wall cavities comes from air leakage, not vapor diffusion through drywall.

What R-value actually measures (and why higher is not always better)

R-value is thermal resistance: how much a material slows the flow of heat. The higher the number, the slower heat moves through. Its inverse, U-value, is the rate of heat transfer per square foot per degree of temperature difference. Both numbers describe the same thing in opposite directions. R-value is what manufacturers print on the packaging, U-value is what engineers use in load calculations.

Heat flow drops with each additional R-value, which is why insulation has diminishing returns. Going from R-0 to R-19 in an attic blocks about 95 percent of the heat transfer that the assembly could ever block. Going from R-19 to R-30 blocks another 2 percent. Going from R-30 to R-49 blocks one more percentage point. Past a certain threshold, the next layer of insulation will not pay for itself before either the materials degrade or the home changes hands. The calculator's payback math makes this explicit so you can see where the curve flattens for your specific climate.

How layered assemblies add up (and where the math gets tricky)

The R-value of a layered assembly is the sum of the R-values of each layer. ASHRAE Fundamentals Chapter 25 calls this the series thermal resistance method. A wall built with half-inch drywall (R-0.45) plus R-13 fiberglass batts plus half-inch OSB sheathing (R-0.5) plus one inch of XPS rigid foam (R-5) plus vinyl siding (R-0.6) sums to R-19.55 by simple addition.

The catch is thermal bridging through framing. Wood studs at 16 inches on center take up about 25 percent of a typical wall surface, and lumber is only R-1.25 per inch versus R-3.2 for fiberglass. The whole-wall R-value of a "R-13 cavity" wall is actually about R-10 to R-11 after the parallel-path framing correction. The simple sum the calculator returns is the nominal value, which matches the number you'll see on building plans and in manufacturer specifications. For real energy modeling, use a 25 percent derate for 2x4 walls and a 23 percent derate for 2x6 walls, or use the full ASHRAE parallel-path method. Continuous exterior insulation (the "R-5 ci" you see in IECC code language) bypasses thermal bridging entirely, which is why it has become standard in code.

Where insulation fits in your HVAC and energy plan

The R-value of your envelope is the largest single input to your heating and cooling load. Doubling the attic R-value from R-19 to R-38 typically cuts attic heat loss by half, which in turn drops your design heating load by 15 to 25 percent depending on how much of total loss the attic represents. That can mean dropping a furnace size or a heat pump tonnage on your next equipment replacement. Run the heat loss calculator with current R-values first, then re-run with the retrofit R-values, and compare the totals.

Insulation also pairs with air sealing. R-value blocks heat flow through the material; air sealing blocks heat flow around it. The air changes per hour calculator uses your blower-door ACH50 result to estimate infiltration loss, which is typically 15 to 35 percent of total residential heat loss. Adding R without sealing leaves most of that on the table. For the financial side, the federal 25C tax credit covers 30 percent of insulation material cost up to $1,200 per year; check your full incentive picture with the rebate finder and frame the return with the payback period calculator.